A system will be developed to monitor the movements of the human mandible efficiently and accurately. The system will consist of a Personal Computer (PC) based data acquisition and analysis unit, a non-restraining extra-oral harness to which is attached a set of Light Emitting Diodes (LED's), and two solid-state detectors to track the LED's in space. Rigorous photogrammetric software will be developed to compute the LED positions in three dimensions, and compute the envelopes of motion of any desired points on the mandible, including the condyle centers. With three LED's mounted on the mandibular portion of the harness and three mounted on the cranial portion, the relative position and orientation of the mandible with respect to the cranium can be determined at rates exceeding 30 times per second. The system will take advantage of recent advances in solid-state detector technology to make such high data rates possible at moderate cost. It will be free of the constraints posed by older mechanical and electromagnetic systems. Such a system would find immediate application in several areas of dental practice. It could be used to measure the changes in physical relationships between the upper and lower jaws during the processes of speech and mastication. Because a detailed mathematical record of the functional movements will be stored, the system could be used to determine functional change after dental restoration, treatment of TMJ disfunction, orthognathic surgery, or orthodontic treatment.